This invention relates generally to a same frequency repeater (SFR) and relates more specifically to an apparatus and method for preventing simulcast distortion in a multiple SFR system.
An SFR is a transceiver that receives a relatively low power RF signal at a particular frequency and rebroadcasts the information carried by the received signal at the same or nearly the same frequency but at a higher power level. An SFR is also referred to as a single frequency repeater. The SFR differs from a conventional repeater which rebroadcasts the received signal at a frequency substantially different from that of the received frequency. A plurality of spaced apart repeaters can extend the area in which a portable or mobile unit can talk-in or communicate with a base station.
In an SFR system in which two or more SFR's can receive a signal transmitted from a mobile unit in an overlapping area of coverage, simulcast distortion may occur. The term simulcast distortion is used to describe interference which occurs when two signals containing the same information, such as voice modulation, are received with one signal delayed in the order of 100 microseconds or longer relative to the other. The audio detected by the base station receiver may exhibit simulcast distortion if two or more SFR's simultaneously rebroadcast a mobile signal because of the time differences which may exist between the two or more retransmitted signals.
Another type of simulcast distortion may occur if a second SFR receives the signal from a first SFR and retransmits same. The audio retransmitted by the second SFR will be time delayed relative to the audio signal transmitted by the first SFR due to the propagation delay between the first and second SFR, and the time required for the signal received by the second SFR to traverse the second SFR. Thus, the base station receiver will receive information transmitted by the second SFR time delayed relative to the same information transmitted by the first SFR thereby resulting in simulcast distortion. Although, precautions can be taken to provide isolation between the SFR's, it is difficult in practice to achieve sufficient isolation to prevent each SFR from receiving a signal transmitted by another SFR.
It is an object of this invention to prevent simulcast distortion in a multiple SFR system by preventing the concurrent transmission by two or more SFR's in the system.
Another object of this invention is to select the SFR having the strongest received signal from a mobile unit for retransmitting the signal.
A further object of the present invention is to provide a means for selecting only one SFR to retransmit a mobile signal wherein several SFR's receive the same mobile signal at approximately the same signal strength.
Another object of the invention is to provide a method for controlling each SFR in a system having multiple SFR's to prevent the concurrent transmission from two or more of the SFR's.